Prince Harry loses case with which he sought to hire private security 0:43

(CNN) -- Prince Harry's years-long battle with Britain's tabloid media will reach its most dramatic stage this week, so far, as Harry prepares to face questions in a London court as part of his case against a major newspaper publisher.

The latest hearing in the Duke of Sussex's case against Mirror Group Newspapers (MGN) began Monday in the High Court. Harry will testify in person on Tuesday, his lawyer told the court.

The appearance of a British royal on a podium will be an exceptionally rare event. But Harry has long railed against tabloid tactics when it comes to covering his life, and now he'll have the opportunity to make his case, under oath, during questioning by MGN's lawyers.

It is likely to be a tense and decisive appearance for the duke, as he is forging his own path, away from the rest of the royal family.

Here's what you need to know about the trial.

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What is the case about?

The Duke of Sussex and three other plaintiffs representing dozens of celebrities have sued MGN, accusing its media of obtaining private information through phone hacking and other illicit means, including private investigators, between 1991 and 2011.

The trial began on May 10 and is expected to last seven weeks.

MGN refutes most of the allegations, claiming in its court briefs that some lawsuits have been filed too late and that in all four cases there is insufficient evidence of phone hacking.

In court documents, published last month, the publisher apologized for a case of illegal information gathering nearly 20 years ago. In 2004, the Sunday People, a tabloid owned by the same group, paid 75 pounds ($95) to a private investigator to gather information about the Duke of Sussex while he was in a London nightclub.

Harry's lawyer, David Sherborne, has said his lawsuit against MGN, which covers incidents from 1995 to 2011, is "significant not only in terms of period, but in the range of activity it covers".

Harry was subjected to the "most intrusive methods of obtaining personal information," Sherborne said, arguing that "no one should be subjected to that." "Illegal methods" were "common and widespread" among journalists, Sherborne added.

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Prince Harry attends the High Court in London 0:42

What will Prince Harry say?

When Harry takes the stand on Tuesday, he awaits harsh questioning by the publisher's lawyers.

The prince alleges that some 140 articles published in newspapers belonging to the group contained information obtained through illegal methods, and 33 of those articles have been selected for consideration in the trial, according to the PA Media news agency.

The details of those articles are likely to be analyzed in depth.

And while it's Harry's first time appearing in a trial against the British media, it may not be his last.

The case against MGN is one of several lawsuits filed by Harry and his wife, Meghan, in their long-running battle against British tabloids, which they have accused of violating privacy and publishing false stories.

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Harry appeared in the High Court in March in a separate case. (Credit: Hannah McKay/Reuters)

The pair have filed at least seven lawsuits against British and American media organizations since 2019, including Rupert Murdoch's News Group Newspapers (NGN), according to Reuters. NGN publishes The Sun and used to produce News of the World, which was shut down in 2011 over a phone hacking scandal.

In March, Harry appeared at a court hearing in his case against Associated Newspapers Limited over allegations of illegal information gathering, which the group has denied.

How will this affect royalty?

Harry's appearance in court is extremely rare. It is believed to be the first time a British royal has appeared in person since 2002, when Princess Anne pleaded guilty after her dog bit two children in a Windsor park, according to PA Media.

It has been more than 130 years since a senior member of the royal family testified in court, when Edward VII was a witness in a trial for slander over a card game in 1891, before becoming king, Reuters reported.

It is not yet clear whether Harry's testimony will refer to other royals or his relationships with other family members. But his brother and heir to the throne, Prince William, has recently been implicated in another of Harry's cases.

Documents released in April as part of Harry's lawsuit against NGN allege that the publisher privately reached an undisclosed settlement with Prince William over historical phone hacking claims. It is unclear how Prince Harry is aware of his brother's agreement, but in his court response he writes that his information is based on a redacted document through which it is inferred that Prince William signed on to the agreement.

Officials at Kensington Palace, which represents Prince William, told CNN it does not comment on legal proceedings. Buckingham Palace reiterated that same stance. Prince Harry claims his late grandmother, Queen Elizabeth II, was aware of talks to reach a deal.

It remains to be seen whether this week's case will raise similar questions for other members of the royal family. But it marks a watershed moment in Harry's efforts against major British media players, and his appearance is likely to dominate headlines in the coming days.

-- CNN's Hanna Ziady, Jessie Gretener, Niamh Kennedy and Sarah Dean contributed to this report.

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